| Root pressure plays an important role in plant water absorption and long-distance transportation, and it is an important mechanism for refilling of xylem embolism in plants.Root pressure is generated by the osmotic potential of the xylem sap and is associated with the physiological activity of roots and the difference of water potential between the inner and outer of root endoderm, as well as osmotic activity of roots.The root pressure is determined by the osmotic potential of xylem sap and the root hydraulic conductivity. In this study, I measured root pressure and its circadian rhythm of excised roots and intact seedlings of 84 K popular(Populus alba x P. glandulosa) under different conditions(different sampling time, different soil temperature, constant/different temperature in day and night), and studied effects of respiratory metabolism and root hydraulic conductivity on the root pressure and its circadian rhythm. I investigated variation of the osmotic potential, circadian rhythm of inorganic ions and soluble sugar contents of root xylem sap and exudation, and diurnal variation of leaf water potential. The influence of defoliation and girdling on root pressure of excised roots and characteristics of stem xylem sap were studied. Based on above experiments, I analyzed and explored the influencing factors and the regulation mechanism of root pressure rhythm. In addition, through testing root pressure and hydraulic conductivity of excised roots from the PtPIP1:1(water channel) gene transgenic 84 K poplar seedlings, I investigated their relationships. We have obtained results and conclusions as follows:(1) Root pressure of excised roots presented diurnal rhythm that was high during daytime and low in the night with 84 K popular seedlings cultured in soil and hydroponics. Different times of sampling and measuring at predawn, noon, and late afternoon had no influence on the rhythm of root pressure. However, the peak value of root pressure was smallest with sampling at noon, then at predawn time, and it was biggest with sampling at late afternoon, indicating that water status of plants could affect root pressure. The soil temperature influenced rootpressure and diurnal rhythm, and the maximum value of root pressure declined with the decreasing soil temperature in the range of 5℃, 10℃, and 20℃. Root respiration rate declined with the decreasing temperature, and the change was synchronized with root pressure,indicating that the decline of root pressure with soil temperature was realized via the root respiration. The root pressure value measured under different temperature in day and night was higher compared to that under day-night constant temperature, which might be related with low-temperature sweetening and sugar accumulating during night. Defoliation, girdling and respiration, root hydraulic conductivity inhibitors had no clear influence on the diurnal rhythm,but defoliation, girdling and respiration inhibitor reduced the maximum values of root pressure significantly(94.22%、75%), while hydraulic conductivity inhibitor had little influence on it.These results indicated that the maximum value of root pressure was mainly influenced by respiration, as well as respiration substance, whereas root hydraulic conductivity had little influence. Diurnal variation of osmotic potential, inorganic ions and soluble sugar content of root exudation was higher during the daytime and lower at night. These results indicated that diurnal rhythm of root pressure was agreed with that of osmotic potential, inorganic ions and soluble sugar content of root exudation, and the ions contributed greater to osmotic potential.(2) Root pressure of intact seedlings could be measured by a sealing pressure transducer to the xylem through a hole tapped on the stem side, and the measurement was approved to be dependable. Under the natural condition, the root pressure of intact seedlings possessed a similar diurnal rhythm with the decapitated root’s, that was higher during the daytime and lower in the night. However, the maximum value of intact root pressure was relative smaller(1.05 kPa) with the influence of transpiration. Diurnal variation of leaf water potential measured under the natural condition showed decline from predawn time while root pressure was raised, and root pressure became to decline only when water potential reduced to a certain degree(-12 kPa). Leaf water potential stayed on a lower level during the afternoon, and it restored to a certain degree(-5 kPa) in late afternoon when root pressure also slightly increased.These results indicated that there existed a positive root pressure during the morning despite the strong transpiration. The root pressure was able to maintain its rhythm of higher duringdaytime and lower in the evening, but the root pressure was decreased by transpiration tension to some degree. Osmotic potential of root xylem sap was lowest, which generated the strongest driving force for water uptake at the predawn time. Osmotic intensity of root xylem sap declined slightly after 6:00 but still maintained a higher level during daytime to refresh osmotic substance taken away by transpiration pull, as well as generated and maintained higher root pressure at the same time. These results illustrated generation of osmotic substance by roots existing strong rhythm. The descending of ions content in xylem sap was accompanied by rising of root pressure during 5:00 to 11:00. Root pressure became to go down when ions content had descent to a certain degree, and the two declined to their minimum value at 15:00all together. Soluble sugar content increased first and then decreased before noon. The peak value of soluble sugar turned up earlier than that of root pressure indicating that soluble sugar was consumed by roots for active transport of ions to maintain root pressure. The content of soluble sugar declined to valley value at 13:00 then kept in fluctuate rising condition and so as root pressure afternoon it decent to the lowest at 15:00. These indicated the component of xylem sap osmotic substance was mainly ions, and soluble sugar played an important role, also.Root pressure was limited to low value all through the afternoon might have relationship with the content of soluble sugar and these needed further research.(3) There existed osmotic potential gradient of xylem sap from top to basal of stem in 84 K popular seedlings. The potential gradient was lowest in the top position, highest in basal position, and the gradient maintained in predawn, noon, and later afternoon, especially significant at noon. This result supported the theory of cohesion. Additionally, osmotic potential of stem xylem sap was lower than that of roots, always. The content of ions and soluble sugar in xylem sap declined from top to basal of stem. The gradient was stable and coincided with that of osmotic gradient, illustrating that the two were the components of osmotic substance of stem xylem sap on the other hand. With decapitation and girdle treatments for one day, ions content of stem xylem sap at predawn time and late afternoon increased significantly in top portion of stem. At predawn time and noon, soluble sugar content increased significantly in top and middle portions of stem. These results illustrated plantswould increase absolute-value of osmotic value of stem xylem sap between top and basal to increase water driving force in response to water loss by transpiration pull and sharp decrease of root pressure to meet water demanding during daytime after the absence of the transpiration and photosynthesis organ. After transporting of organic substance to roots was inhibited, plants would increase absolute-value of osmotic value of stem xylem sap between top and basal during afternoon to increase water driving force in the evening to complement water lost under transpiration pull in the daytime.(4) Fore transgenic genotypes(over expression of PtPIP1; 1, PB11; suppress expression of PtPIP1; 1, PR11; over expression of PtPIP1; 1 in vascular tissue specially, TPB11; and suppress expression of PtPIP1; 1 in vascular tissue specially, TPR11), and wild 84 K poplar seedlings were used to investigate the relationship between root pressure and root hydraulic conductivity. The results showed as follows: root pressure of excised roots from all genotypes performed higher during the daytime and lower in the night. The maximum value of root pressure of TPB11 and WT seedlings was greatest and that of TPR11 seedlings was smallest.There existed no significant difference between maximum root pressure of PB11/PR11 and WT seedlings, but there existed significant difference between that of TPB11/WT and TPR11 seedlings. These results indicated that PIP1:1 had no impact on rhythm of root pressure and it might influence maximum root pressure together with other factors. Root hydraulic conductivity of WT seedlings was the maximum while that of all other genotypes was smaller,and there was no significant difference in root hydraulic conductivity among those transgenic genotypes. These results suggested that there was no significant correlation between root hydraulic conductivity and root pressure. |
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